1. Field of the Invention
The present invention relates to the removal of arsenic, iron, phosphorus, sulfur, manganese and other dissolved minerals, whether naturally occurring or byproducts of commercial activities and, more particularly, to conversion of arsenic to arsenate and oxidation of other organic and inorganic matter.
2. Description of Related Prior Art
Arsenic occurs naturally in the environment as a heavy metal in two different forms, arsenite [As (III)] and arsenate [As (V)]. Arsenic is released into water supplies from erosion of rocks and soil. The distribution of arsenic in soil, ground water and surface water has extensively been investigated during the past two decades.
Long term exposure is proven to result in health effects, such as cancer, cardiovascular disease, diabetes and reproductive problems. Nationally, about three thousand or 5.5% of the nation's 54,000 community water systems and 1,100 (or 5.5%) of the twenty thousand non-transient non-community water systems will need to take measures to lower arsenic in their drinking water. Of the affected systems, 97% serve fewer than 10,000 people. While high concentrations of arsenic are found mostly in the western region of the United States, parts of the mid-west and New England show levels of arsenic that exceed the newly approved U.S. Environmental Protection Agency (EPA) standard of ten (10) parts per billion (ppb). The western states have more systems greater than ten (10) ppb as compared to the national average. Some systems and part of the mid-west and New England have current arsenic levels that are greater than ten (10) ppb but most systems have arsenic levels that range from two (2) to ten (10) ppb of arsenic.
Jan. 23, 2006 was the EPA drinking water compliance deadline for the Revised Arsenic Standard to ten (10) ppb Maximum Contaminant Limit (MCL). The revised standard was issued in light of the myriad of both the short term and long term health effects of ingested arsenic in drinking water. Short or acute effects can occur within hours or days of exposure. Long or chronic effects occur over many years. Long term exposure to arsenic has been linked to cancer of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate. Short term exposure to high doses of arsenic can cause other adverse health effects, but such effects are unlikely to occur from U.S. public water supplies that are in compliance with the arsenic standard.
A variety of technologies have been developed for arsenic removal. All of the technologies rely on a few basic chemical processes. These include:                1. Oxidation/reduction reactions that reduce (add electrons to) or oxidize (remove electrons from) chemicals and altering their chemical form. These reactions do not remove arsenic from solution but are often used to convert the trivalent form of arsenic (As III) to the pentavalent form (As V). The latter form is much less toxic and is more readily removed via precipitation, ion exchange and adsorption than is the former.        2. Precipitation: causing dissolved arsenic to form a low solubility solid mineral, such as calcium arsenate. This solid can then be removed through sedimentation and filtration. When coagulants are added and form flocculants, other dissolved compounds such as arsenic can become insoluble and form solids which is known as co-precipitation. The solids formed may remain suspended and require removal through solid/liquid separation processes, typically coagulation and filtration.        3. Adsorption and ion exchange: various solid materials, including iron, titanium and aluminum hydroxide flocculants, have a strong affinity for dissolved arsenic. Arsenic is strongly attracted to sorption sites on the surfaces of these solids and is effectively removed from solution. Ion exchange can be considered as a special form of adsorption, though it is often considered separately. Ion exchange involves a reversible displacement of an ion adsorbed into a solid surface by a dissolved ion. Other forms of adsorption involve stronger bonds and are less easily reversed.        4. Solid/liquid separation: precipitation, co-precipitation, adsorption and ion exchange all transfer the contaminant from the dissolved to a solid phase. In some cases the solid is large and fixed (e.g. grains of ion exchange resin) and no solid/liquid separation is required. If the solids are formed in situ (through precipitation or coagulation), they must be separated from the water. Gravity settling (also called sedimentation) can accomplish some of this but filtration is more effective. More commonly, sand filters are used for this purpose.        5. Physical exclusion: some synthetic membranes are permeable to certain dissolved compounds but exclude others. These membranes can act as a molecular filter to remove dissolved arsenic, along with many other dissolved and particulate compounds.        6. Biological removal processes: bacteria can play an important role in catalyzing many of the above processes. Relatively little is known about the potential for biological removal of arsenic from water. While all of these techniques have merit, some of the most effective approaches involve oxidation. In March 2001, Ganesh Ghurye and Dennis Clifford of the University of Houston studied seven different oxidants for the removal of arsenic in an EPA sponsored study (Contract No. 8C-R311-NAEX) entitled “Laboratory Study on the Oxidation of Arsenic III to Arsenic V”.Their conclusions were:        Free chlorine: dissolved manganese, dissolved iron, sulfide, and Total Organic Carbon (TOC) slowed the rate of oxidation slightly but essentially complete oxidation of arsenite As (III) to arsonate As (V) was obtained in less than one minute.        Chloramines: preformed monochloramine was ineffective for As (III) oxidation.        Chlorine dioxide: only limited oxidation was achieved with chlorine dioxide.        Permanganate: dissolved manganese, dissolved iron, sulfide and TOC slowed the rate of oxidation slightly but essentially complete oxidation of As (III) to As (V) was obtained in less than one minute.        Ultraviolet (UV) radiation: UV (254 nm) was not very effective.        Oxidizing media: manganese dioxide media showed effectiveness when dissolved oxygen levels were not limiting. Interfering reductants led to incomplete oxidation.        Ozone: in the absence of interfering reductants, such as sulfide and TOC, ozone rapidly oxidized As (III). TOC had a quenching effect at higher TOC levels but the presence of sulfide considerably slowed the oxidation of As (III).        
Ghurye and Clifford reported that ozone performed the best provided that sulfides and organic matter were not present in the water as these compounds created an ozone demand that interfered with the ability of ozone to convert the As (III) to As (V). Ozone is much preferred over chlorine and permanganate if the objective is to reduce chemical use.